The invention relates to a high-pressure sodium discharge lamp comprising a discharge envelope which is provided with a filling containing sodium and a rare gas, and at least one current-supply conductor which is passed through the wall of the envelope to a solid electrode consisting mainly of tungsten and free of alkaline earth metals. Such lamps, which are known from Dutch patent application No. 8005025, to which co-pending U.S. application Ser. No. 694,155 (assigned to the same assignee as the instant application) corresponds, have the advantage of a high luminous efficacy. The discharge envelope of such lamps consists of a crystalline oxide resistant to sodium vapour, such as, for example, mono-crystalline sapphire or densely sintered polycrystalline aluminum oxide. The filling of the discharge envelope may contain mercury in addition to the sodium and one or more rare gasses.
It has been found that in the known lamp in many cases the electrode is attacked, which in the end causes the electrode to break off, sometimes after an operating time of approximately 1000 hours. This causes the actual life of these lamps to be seriously less than the envisaged life of at least 2000 hours.
In literature a sintered electrode containing tungsten and rhenium has been suggested for a high-pressure discharge lamp. The electrode then further contains tantalum carbide. However, experiments have shown that under conditions prevailing in high-pressure sodium discharge lamps tantalum carbide gives rises to a rapid blackening of the discharge envelope due to sputtering and evaporation. Moreover, an electrode, which is not solid, but is sintered, has the property that sputtering will occur more readily than in a solid electrode.